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By default STELLAR calculates the capacitances between
interconnect lines and the whole substrate. In certain conditions it may lead
to some inaccuracies. For example STELLAR may calculate
the capacitance of a poly line over an active area but this capacitance is already
present in the spice compact model (Cox). Another example is that the substrate
of a MOSFET transistor is usually connected to Vdd or Gnd depending on its polarity.
So we may need to separate the Vdd areas from the grounded substrate. The aim
of the substrate partition feature is to divide the substrate in different regions
to calculate or not capacitances between interconnect lines and these different
substrate regions. For example, we do not want to calculate the capacitance
between a line over the substrate region shown in Figure 1.

Figure 1. Final mask layers defined in STELLAR.

STELLAR GUI

Techno file: layer setup
We will explain here how to define
the mask layers shown in Figure 1. The NWELL, PACT, NACT regions are defined
in the original gds file (Figure 2).

Figure 2. Original mask layers in the gds file. .

The other layers are defined in the technology panel of STELLAR

BULK is defined as a « Derived Layer » (Figure 3).

Figure 3. BULK derived layer definition.

SUBS is a key word in STELLAR which defined the total
surface of the substrate (actually the smallest area including all the layout).
The NWELL1, BULK1 are defined in the STELLAR GUI also
as « Derived Layer » (Figure 4 and 5).

Figure 4. NWELL1 derived layer definition.

Figure 5. BULK1 derived layer definition.

By default STELLAR will name « Substrate »
all the substrate region which does not belong to a defined region (Figure 1).

Techno file: connection setup
The connection is defined in the STELLAR techno panel.
If there are layers that have to be connected to the substrate (CONT_BULK, CONT_WELL)
then we have to use the connectivity panel as indicated in Figure 6.

Figure 6. Substrate connectivity.

Process Definition

The layers NWELL1, NBULK1 have to be defined in the Process panel of STELLAR
using a specific key word: « Substrate_Partition » (Figure
7).

Figure 7. substrate partition definition.

From the « Substrate_Partition », STELLAR
builder will generate a plane surface with a thickness=0.

Note that labels attach to the substrate_Partition is the name of that layer
used in the process panel. For example in Figure 7 we use BULK1 as a substrate
layer and STELLAR will output in the spice netlist
the capacitance between BULK1 and poly.

Example
We take an example as shown below :

The capacitance calculation without the substrate partition gives:

Final capacitance report ( in F )- sorted

poly

substrate

1.471416e-14

The capacitance calculation with the substrate partition gives:

Final capacitance report ( in F )- sorted

poly

substrate

1.093147e-14

BULK1

poly

1.901284e-15

NWELL1

poly

1.881415e-15

Application: Hiding layer

We want to calculate capacitances using the substrate partition feature of
STELLAR and compare to CLEVER
for validation purpose. The layout used for this study is shown in Figure 8.

Figure 8. Layout under study.

There are 3 mask layers identified in the STELLAR
techno panel (Figure 9). We want to calculate the capacitance between the Metal1
plate and the substrate without taking into account the overlap capacitance
between the Metal1 plate and substrate. Note that layer 2, named hiding_layer,
was specifically design in order to define the substrate partition. Indeed GND
layer is defined in the techno panel (Figure 9) as a derived layer: GND=substrate
- hiding_layer and will be used in the process panel as a substrate_Partition
(Figure 10).

Figure 9. Layer setup definition.

Figure 10. Process panel showing the use of GND layer
as a substrate partition layer type.

Note that from the « Substrate_Layer », (GND) STELLAR
builder will generate a plane surface with a thickness=0 as shown in Figure 11.

Figure 11. 3D structure without oxide.

The capacitance calculation with the substrate partition gives:

Final capacitance report ( in F )- sorted

plate

GND

4.81e-15

The capacitance calculation without the substrate partition gives:

Final capacitance report ( in F )- sorted

plate

substrate

3e-14

In this last case the overlap capacitance between the plate and the substrate
was calculated increasing significantly the overall capacitance.

Comparison with CLEVER

We wanted to compare STELLAR results with CLEVER
for validation purpose. For that we obviously used the same layout (Figure 8)
and the same process. What we did in CLEVER to mimic
the substrate partition feature used in STELLAR is
shown in Figure 12. We use the metal1 mask to define a region in the substrate
that we fill with a dielectric and on top of that we set a very low permittivity
for this dielectric. The resulting 3D structure is shown in Figure 13.

The capacitance calculation for the standard structure gives:

Final capacitance report ( in F )- sorted

plate

substrate

3.08e-14

The capacitance calculation in CLEVER which mimic the STELLAR substrate partition:

Final capacitance report ( in F )- sorted

plate

GND

4.59e-15

The difference between CLEVER and STELLAR
results are less than 5%.

Conclusion

We have shown that STELLAR is able to define different
substrate regions. This feature is useful to avoid the calculation of some capacitances
like Cox already present in the spice compact model or to separate the Vdd areas
from the grounded substrate.